1. Field of the Invention
This invention relates to guns for mixing and dispensing plural component plastic materials, for example, polyurethane foam, and particularly to an improved arrangement for purging residue from the mixing chamber.
2. Description of the Prior Art
The purging device of this invention relates to plural component plastic application equipment or guns of the general type described and claimed in U.S. Pat. Nos. 3,366,337 and 4,133,483 wherein two coreactive streams of chemicals are intimately mixed within the gun and dispensed as a fluent stream or spray. The present invention will be described in connection with plural component foam producing chemicals, but is understood to be generally applicable to mixing guns of the general character irrespective of the particular chemicals used.
One problem which arises in prior art foam guns is that, between operations, the residual material in the mixing chamber and/or outlet nozzle tends to harden and jam the outlet thereby making further mixing and dispensing impossible until the chamber and nozzle are purged of the hardened residue. Purging is accomplished, for example, in the aforementioned U.S. Pat. No. 3,366,337, by manually opening a needle valve to admit a flow of solvent and/or air through the chamber and nozzle to remove the residue.
This purging procedure had several disadvantages. First, the basic operation required the use of two hands because the needle was opened and closed by a threaded knob. Second, the use of solvent added unnecessary expense to the foam process. Further, the solvent, because of being ejected under pressure, was dispersed into the surrounding air and created a potential safety and health problem. Still further, the blast of solvent and/or air, while forcefully ejecting the hardened foam, scattered the residue haphazardly. Overall, this cleaning arrangement was inefficient.
Other prior art arrangements, such as for example Gusmer U.S. Pat. No. 3,263,928 and Gusmer et al U.S. Pat. No. 2,890,836, in an attempt to overcome the mentioned difficulties, utilize a rod to eject excess material between spraying operations. In these devices, the rod purports to purge the mixing chamber and outlet nozzle on a single stroke each time the gun is shut off. However, such rods were designed to scrape the walls of the chamber, were utilized as the valving mechanism to open and close the infeed orifices and, for this reason, could be utilized for only one purging stroke for each dispensing operation. In practice, it was found that on the return stroke the rod tended to pull back some of the residue. If the gun was then left unused for a period of time, the residue hardened and bonded causing the gun to jam. For this reason, such guns utilized a solvent bath giving rise to the same problems mentioned heretofore relative to other prior art foam guns.
In the aforementioned U.S. Pat. No. 4,133,483, purging is accomplished through a purging rod operable separate from and independently of the foam mixing and dispensing means of the gun. The purging rod is not utilized to open and close the inlet ports for the chemicals which form the foam. Thus the gun may be used several times without using the purging device. However, use of solvents to insure complete removal of foam residue is still necessary.